Molecular simulation of polymeric nanoparticles

Engineering functional polymeric nanoparticles requires an understanding of their crystallization effected by particle size, surface effects, and other processing conditions.

Nanoparticles are often crystallised in isolated form or adjacent to surfaces in many processing situations, such as atomization and spray drying. Understanding these processes is also essential from a theoretical perspective, where the notion of homogeneous crystallization breaks down in the presence of surface effects. How does the crystallization of a polymeric nanoparticle differ from that of a bulk material? How does surface and its topological feature affect the results? What are the effects of particle size on the kinetics of crystallization and growth mechanism of crystallinity? What are the spatial-temporal structural and physical properties? These are some of the questions we try to answer in this research.

Molecular simulations are powerful means to investigate phenomena at the nanoscale. Here, molecular simulations are used to study the interaction of polymeric nanoparticles with surface and in a vacuum, and the surface effects are studied. The processes considered here are relevant in many industries where cold and hot spray deposition techniques are used for coating or nanoparticle production. Underlying surface-polymer interfaces are also vital in the manufacture of composite materials, and crystallization of a polymer near surfaces is of interest to many applications. We have recently investigated the effect of surface roughness on wetting [1] and the crystallization kinetics [2] of polymeric nanoparticles, and have shown that depending on the size of roughness, crystallization can be impeded by the roughness.

Click to enlarge. Degree of crystallinity as a function of time for a polymeric nanoparticle in vacuum (a), entirely (b) and partially (c) diffused onto cavities on a rough surface, and on an atomically smooth surface (d).

In this study, the diffusion and crystallization of hexacontane – a relatively long alkane as an isolated drop and as a drop on smooth and rough surfaces – were studied using molecular dynamics simulations. By adjusting the lattice size of the underlying crystal structure, we produced substrates of varying degrees of oleophilicity and created a surface that allowed easy diffusion of the polymer nanodrop on a smooth surface. The smooth surface was subsequently roughened with square pillars creating surfaces with rectangular grooves with different gap widths (w) and roughness ratios. The effect of surface roughness on the crystallinity the of polymeric nanoparticle was investigated by varying the size of the surface features. The crystallinity was measured for a nanodrop which was partially penetrated inside the grooves (Wenzel state) and for a drop which fully penetrated the grooves.

In both cases, the rough features impeded the crystal growth. The maximum degree of crystallinity was shown to reduce by reducing the gap width and size of the surface features. This was consistent with the experimental results with for polyethylene crystallization on silicon surfaces [3]. Crystallization of hexacontane nanodrop for four different states was compared, including an isolated drop in a vacuum, on a smooth oleophilic surface, and on rough surfaces for Wenzel and fully confined states. Significant differences were found for these modes of crystallization: the isolated drop in the absence of any surface showed the slowest crystallization. By contrast, a polymer drop that spreads on a smooth surface had the largest degree of crystallinity and crystallized the fastest, demonstrating surface-induced crystallization.

It was demonstrated that introducing roughness in the form of square pillars on the surface impedes stretching and folding of the molecules, leading to a lower degree of crystallinity for the polymeric nanoparticle on rough surfaces.

Hexacontane nanoparticle spreading

A ~15 nm polymeric nanoparticle released near an atomically smooth surface with strong wettability results in spreading. The movie is captured by molecular simulations and show the process from the side and top views.